Device and method for filtering a fluid, in particular for plastics processing plants

ABSTRACT

The invention relates to a device for filtering a fluid, in particular a liquefied plastic, with a housing with backflush channel and screen carrier and filter element whereby, in order to accelerate and improve the backflush effect, one or more displacement cylinders are provided that guide material removed from the production at adjustable high pressure and speed to the screen to be cleaned from the clean screen side to the dirty screen side, whereby an additional pressure generator ensures constant process pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for filtering a fluid, in particular aliquefied plastic, with a housing with a supply channel, a dischargechannel and a backflush channel, whereby in the flow path of the fluid,in a screen carrier slideably supported crosswise to the flow direction,two filter elements are arranged in corresponding screen chambers andcan be linked to the supply channel and the discharge channel.

2. Discussion of Background Information

Devices for filtering a fluid are known, e.g., from DE 35 27 173 C1.With these known devices, the arrangement is made such that two screencarriers are arranged parallel to one another so that the supply of thedevice connected to the filter device does not need to be disconnectedduring the flushing of the screen chambers of the one screen carrier.The melt free of contamination used for flushing purposes is removeddownstream of the screen carrier just now in the operating position. Thebackflush effect is essentially determined by the flow rate of thebackflush flow, which for its part again depends on the back pressure ofthe downstream device connected to the filter device. This back pressureis generally relatively low and cannot be influenced at will. Thisresults in a poor or, primarily in the case of larger screen surfaces,only partial flushing clean of the soiled screen. Moreover, a loss ofmass occurs in the main flow at the time of the backflushing, which lossof mass is greater, the longer time is necessary for an adequateflushing of the screen chamber or the screen of the screen carriercurrently in the backflush position.

WO 92/16 351 A1 discloses a filtering device for fluids to be purifiedthat operates with an accumulator piston acting in a cylinder. In orderto make the rinsing operation quick, effective and uniform over thesurface of the screen and to avoid additional loss of mass in the mainflow during backflushing, there is a reservoir in the screen carrierbody. The reservoir is filled with purified melt during normal operationand which an accumulator piston can move. In the backflushing positionof the screen carrier body the melt located in the reservoir is forcedthrough the screen and into the backflush channel in the backflushingdirection. The use of an additional cylinder chamber and accumulatorpiston is laborious and always carries the risk that plastic residueswill remain in the chamber processed by the accumulator piston, whichresidues can crack and thus cause problems.

With this known device it is therefore necessary for the reservoirpresent in the screen carrier body that is acted on by the accumulatorpiston to be filled with purified melt. At the time this storage chamberis filled, a pressure loss naturally occurs at the discharge channel orin the equipment or tool attached to the filter device, so that theprocess pressure is not constant.

A device is known from DE 17 88 037 U1 for separating mixturescontaining liquid and solid matter, in which the liquid containing solidmaterial is guided through a screen, whereby an accumulator piston ismoved back at the same time. A discharge line is connected to the areaof the accumulator piston, through which line the purified liquid isdischarged. The connection between the accumulator piston and thedischarge line is equipped with a check valve that opens when theaccumulator piston generates a corresponding pressure in the dischargeline. At the same time, the purified liquid can also be pressed throughthe screen again with the movement of the accumulator piston, in orderto clean the screen of trapped solid particles. A device of this typecannot be used for liquefied plastic, in particular because in thedischarge channel the check valve provided there would very quickly clogand become unusable. Moreover, material being retained in the areabetween the check valve and accumulator piston would disintegrate, i.e.,dead zones are also present here which definitely must be avoided inprocessing liquid plastic.

SUMMARY OF THE INVENTION

The invention is directed to a method and a device in which thebackflushing effect can be greatly improved, yet at the same time theloss of mass in the main flow at the time of the backflush is alsoavoided. The displacement piston arranged in the channel is alwaysflushed around by material so that no material can decompose here andthe pressure at the screen carrier housing outlet can be kept constant.

This is attained through the teaching of originally filed claim 1defining the device and with a method with the features of originallyfiled claim 10.

According to this teaching the so-called displacement pistons areassigned to the partial channels leading from the screen chamber orchambers to the discharge channel. These partial channels are thusembodied straight and relatively long, and a displacement piston opensinto these partial channels which, in the backflushing position, feedsthe contaminant-free material located in this partial channel to theback of the filter element, namely in turn at a pressure that is muchhigher than the pressure that normally prevails on the clean screen sidein the production position of the screen carrier. In this manner, anadditional pressure generator ensures a constant process pressure in thesupply channel, even when the displacement piston is drawn back.

Furthermore, the invention provides that the screen carrier can bedisplaced such that the filter elements can be removed from the screenchambers.

Connection plates are preferably connected to the housing accommodatingthe screen carrier, whereby the displacement pistons are more easilyreplaceable and the channels are easier to produce.

The displacement piston or pistons can be acted on hydraulically,mechanically, e.g., via toggle lever systems, toothed racks, etc.

A flow divider can be housed in the supply channel and a flow divertercan be housed in the discharge channel in order to thus create good flowconditions.

The operating pressure of the displacement piston or pistons can bepreset.

Finally, in the invention a backflushing method with a device forfiltering a fluid, in particular a liquefied plastic is provided. In thebackflush position of the screen carrier the screen in the backflushposition is acted on via displacement pistons with a pressure that actsexclusively in the direction of the backflush flow and which can be muchhigher than the operating pressure prevailing on the production side ofthe screen. Also, a constant process pressure can be maintained via apressure generator in the supply channel.

The device according to the invention and the method according to theinvention have the great advantage that the backflushing occursimmediately as soon as a corresponding demand signal occurs. Also, aconstant process pressure prevails in the connected apparatus or tool.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is explained below on the basisof the drawings. The drawings show:

FIG. 1 shows a device with two displacement pistons; and

FIG. 2 shows a section through the screen carrier with a deviceaccording to FIG. 1.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIGS. 1 and 2 show a housing 1 accommodating a screen carrier 4, wherebya supply channel 2 and a discharge channel 3 are arranged in the housing1. A connection plate 18 connects to the side of the screen carrier 4facing downstream.

Filter elements embodied as screens 5, 6 are provided in screen chambers7, 8 in the screen carrier 4. It should be pointed out that the term“screen carrier” is used in connection with the term “screen,” however,the term “screens” also relates to the different types of screens,filters and other retaining devices for contaminants.

A flow divider 19 is discernible in the supply channel 2 (FIG. 2) and apressure generator 24, e.g., a pump, is arranged in front of the supplychannel 2.

Furthermore, backflush channels 9 and 10 that open into the atmosphereare provided in the housing 1.

Partial channels 20, 21 are embodied in a straight line and accommodatedisplacement pistons 22 and 23 that can be activated via a mechanicaltappet device in the arrangement shown in FIG. 1. In the arrangementaccording to FIG. 1, a connection plate 18 is discernible, through whichthe displacement pistons 22, 23 can be more easily replaced in the eventof damage or the like.

FIG. 2 shows a section in the longitudinal axis of the screen carrier 4.FIG. 2 also shows the screens 5 and 6 and the screen chambers 7 and 8,as well as the partial channels 20 and 21.

To return to the representation in FIG. 1, the displacement piston 23 isin a position in which the screen 5 in the screen carrier 4 produces theconnection between the supply channel 2 and the discharge channel 3,i.e., the device is thus in the production position.

In contrast, the displacement piston 22, however, is moved downward and,moreover, the screen 6 located in the screen carrier 4 a is connected tothe backflush channel 9 through displacement of the screen carrier 4 a.In this position, the displacement piston 22 guides the clean materiallocated in the partial channel 20 assigned thereto with adjustable,relatively high pressure to the clean screen side and thus freed fromdirt the dirty screen side of the screen 6 located in the screen carrier4 a.

In the arrangement according to FIGS. 1 and 2, the end face of thedisplacement pistons 22 or 23 is always flowed around by material sothat no dead zones occur in which material can decompose.

The pressure generator 24 additionally arranged in the area of thesupply channel 2 is preferably a pump, but other pressure generators canalso be used. The pump or other pressure generators ensures that,regardless of the backflush, i.e., regardless of whether both channels20 and 21 are in use or only one channel, and regardless of where thedisplacement piston 22 or 23 is in channel 20 or 21, the processpressure in the area of the discharge channel 3 is always kept constant.This is very important for the equipment or tool connected to thedischarge channel 3.

1-12. (canceled)
 13. A device for filtering a liquefied plastic,comprising: a housing; a supply channel; a discharge channel; backflushchannels; two screen carriers slidably supported crosswise to a flowdirection; at least two filter elements each of which are arranged incorresponding screen chambers and linked to the supply channel and thedischarge channel; at least two displacement pistons being inserted in acorresponding partial channel leading from the screen chamber to thedischarge channel, the at least two displacement pistons feed a massfree of contaminants out of the corresponding partial channels throughthe at least two filter elements to a corresponding backflush channel ofthe backflush channels when at least one of the two screen carriers isin the backflush position; and a pressure generator arranged in thesupply channel, the pressure generator being structured to maintain aconstant process pressure at the discharge channel and downstreamequipment during a production phase.
 14. The device according to claim13, wherein the pressure generator is a pump.
 15. The device accordingto claim 13, wherein the screen carriers are displaceable such that theat least two filter elements can be removed from the screen chambers.16. The device according to claim 13, further comprising connectionplates attached to the housing and accommodating the screen carriers.17. The device according to claim 13, wherein at least one of thedisplacement pistons are activated hydraulically.
 18. The deviceaccording to claim 13, wherein at least one of the displacement pistonsis activated mechanically.
 19. The device according to claim 13, furthercomprising a flow divider arranged in the supply channel in front of thescreen chambers.
 20. The device according to claim 13, furthercomprising a flow diverter arranged in the discharge channel.
 21. Thedevice according to claim 13, wherein the operating pressure for thedisplacement pistons is adjustable.
 22. The device according to claim13, wherein at least one of the two displacement pistons is structuredto load a clean screen side with purified plastic melt when a respectiveof the two screen carriers is in the backflush position.
 23. A devicefor filtering a liquefied plastic, comprising: a housing accommodating ascreen carrier, a supply channel and a discharge channel; backflushchannels provided in the housing which are open into atmosphere; partialchannels arranged between the supply channel and the discharge channel;displacement pistons arranged in the respective partial channels; and apressure generator arranged in the supply channel, the pressuregenerator being structured to maintain a constant process pressure atthe discharge channel and downstream equipment during a productionphase.
 24. The device according to claim 23, further comprising amechanical tappet device structured to activate the displacementpistons.
 25. The device according to claim 23, further comprising aconnection plate connecting to a side of the screen carrier facingdownstream.
 26. The device according to claim 23, wherein the pressuregenerator is a pump.
 27. The device according to claim 23, wherein thepressure generator is structured such that regardless of a backflush aprocess pressure in an area of the discharge channel always is keptconstant.
 28. The device according to claim 23, wherein filter elementsare screens in screen chambers in the screen carrier.
 29. The deviceaccording to claim 23, further comprising a flow divider in the supplychannel.
 30. The device according to claim 23, wherein an end face ofthe displacement pistons is always flowed around by material so that nodead zones occur in the material can decompose.
 31. The device accordingto claim 23, wherein one of the displacement pistons is in a position inwhich a screen in the screen carrier produces a connection between thesupply channel and the discharge channel.
 32. A method for backflushinga liquefied plastic, comprising: loading a clean screen side of a filterelement with purified plastic melt with a displacement piston when ascreen carrier is in a backflush position; plunging the displacementpiston into a partial channel leading from a supply channel to adischarge channel when the screen carrier is in the backflush positionand forcing the plastic melt out of the partial channel through thefilter element in the backflush position; acting on the filter elementwith a pressure that is higher than an operating pressure prevailing ona production side of the filter element; and maintaining a constant meltpressure at the discharge channel via a pressure generator arranged inthe supply channel.
 33. The method according to claim 32, wherein thebackflush pressure is adjustable.
 34. The method according to claim 32,further comprising controlling the melt pressure depending on a pressurevalue measured at the discharge channel.